1. Field of the Invention
The invention relates to a mercury lamp of the short arc type. The invention relates especially to a mercury lamp of the short arc type with high focusing efficiency and good light intensity stability which is used for a semiconductor exposure device.
2. Description of Related Art
Recently, in the exposure process in the manufacture of semiconductors, a mercury lamp of the short arc type has been used which emits UV radiation with a primary wavelength of 365 nm (hereinafter called the xe2x80x9ci-linexe2x80x9d). Since the degree of integration of an integrated solid-state circuit increases each year, there is a greater and greater demand for higher image resolution during exposure. Furthermore, due to the increase in the exposure surface as a result of increasing the wafer aperture or due to the modified illumination which is used to achieve a high image resolution, there is a demand for increasing the amount of UV radiation emitted from the light source (which is hereinafter called only the xe2x80x9camount of radiationxe2x80x9d).
Furthermore, there is also a demand for an increase in the throughput as indicated by the amount of production per unit of time. Therefore, there is a demand for high radiation efficiency in using the lamp as a light source, while at the same time, there is a demand for high focusing efficiency in using the lamp as an emission device.
To obtain intensive i-line radiation, conventionally, a process is used in which the input power supplied to the lamp is increased. However, when the input power supplied to the lamp is increased, the thermal burden on the electrodes also increases, thereby causing more vigorous vaporization of the electrode material and accelerating the blackening of the arc tube. Furthermore, by increasing the input power, the arc tube is required to have a larger outside dimension since a larger air blower device is needed to dissipate the heat produced by the lamp. Therefore, a process in which more intensive i-line radiation is obtained by increasing the input power supplied to the lamp is not desirable.
Therefore, a primary object of the present invention is to devise a mercury lamp of the short arc type in which an arc tube can be kept unfouled for a longer time and intensive i-line radiation can be obtained.
A further object of the invention is to devise a UV emission device which will effectively obtain the above object.
In a first aspect of the invention, these objects are achieved by a mercury lamp of the short arc type is provided including a cathode and an anode disposed opposite one another within a quartz arc tube filled with mercury and a rare gas of at least argon (Ar) or krypton (Kr) with a pressure from 1.0 to 8.0 atm are added at room temperature, and by satisfying the condition 0.3xe2x89xa6X/Lxe2x89xa60.6, where L is the length of the arc tube in the axial direction in millimeters and X is the length of the cathode in millimeters which projects in the axial direction into the emission space, the direction between the anode and the cathode being defined as the axial direction.
In another aspect of the invention, these objects are achieved in a mercury lamp of the short arc type which corresponds to the lamp described in the preceding paragraph by adding the rare gas as a gas mixture of argon and krypton with a total pressure from 1.0 to 8.0 atm at room temperature instead of adding argon or krypton separately.
In accordance with yet another aspect of the invention, these objects are achieved in a mercury lamp of the short arc type which corresponds to the lamp described above by satisfying the conditions 0.85Dxe2x89xa7(X+5) and 0.85Dxe2x89xa7Lxe2x88x92(X+5), where D is the maximum outside diameter of the arc tube in the radial direction in millimeters, the radial direction being defined as the direction of the cross section which is perpendicular to the axial direction of the arc tube.
The objects of the invention are furthermore achieved by a UV emission device which comprises:
a mercury lamp of the short arc type described above, and
a power source which supplies a predetermined power to the mercury lamp.
Advantageously, in a UV emission device, the objects are furthermore achieved by the mercury lamp of the short arc type being arranged vertically such that the anode is at the top and the cathode is at the bottom.
The mercury lamp of the short arc type in accordance with the present invention is characterized in that at least argon (Ar) or krypton (Kr) or a mixture of these gases underpressure is added as the buffer gas. It was possible to confirm by tests that the above described measure prevents broadening of the spectral width of the i-line. Therefore, it was possible to ascertain that the irradiance of the exposure surface correspondingly increases. The reason for this is that the radiation efficiency increases by 10 to 20% as compared to adding xenon gas (Xe) at roughly 1 atm (this increased amount of efficiency corresponds to an increase of 20 to 40% when it is converted to power).
Furthermore, the invention is characterized by the fact that under the condition that at least argon (Ar) or krypton (Kr) or a mixture of these gases under pressure is added as the buffer gas with a predetermined pressure, the relationship between the various dimensions of the arc tube (hereinafter also called the xe2x80x9cbulbxe2x80x9d) is fixed such that each of the dimensions of the bulb are dependent upon the other dimensions of the bulb. The expression xe2x80x9cdimensions of the bulbxe2x80x9d is defined specifically as the values of length of the bulb in the axial direction, the length of the cathode which projects in the axial direction into the emission space, and the maximum outside diameter of the bulb in the radial direction, the axial direction being defined as the direction between the anode and cathode, and the radial direction being defined as the direction of the cross section which is perpendicular to the axial direction.
The reason for fixing the relationship between the various dimensions of the bulb such that each of the dimensions of the bulb is dependent upon the other dimensions of the bulb lies in the fact that the added buffer gas largely determines the thermal behavior within the bulb, and therefore, exerts strong effects on the thermal behavior and the arc characteristic within the bulb when the molar ratio of the added buffer gas to the simultaneously added mercury is large.
The reason for the above described dependencies between the dimensions of the bulb is not entirely clear, but presumably lies in the following:
It can be imagined that the reason for the described dependencies lies in the different thermal conductivities of the Ar gas, the Kr gas and the Xe gas. If this thermal conductivity is high, the rate of transfer of the thermal energy increases. The temperature in the arc center is easily transferred into the vicinity of the inner bulb surface, while conversely, the temperature of the vicinity of the inner surface of the bulb is easily transferred to the arc center. In this case, the thermal conductivities of the Ar gas, the Kr gas and the Xe gas (10xe2x88x924 W/cm/K) are in the sequence (Ar: 1.63) greater than (Kr: 0.88) greater than (Xe: 0.50). The mercury lamp in which Ar gas or Kr gas is added is more easily influenced by air blowout cooling of the outside surface of the bulb and similar conditions than a mercury lamp filled with Xe gas. This leads to a temperature drop in the vicinity of the inner surface of the bulb and to a temperature drop in the arc center.
The different thermal conductivities exert major effects not only on the temperature drop in the arc center, but also on gas convection within the emission space. The stability of the arc thereby depends on this gas convection. When undesirable gas convection occurs, the arc stability is adversely affected which can also lead to formation of fluctuations.
The arc fluctuations formed by the temperature drop in the arc center or the undesirable gas convection cause fluctuations and nonuniformity of the illuminance of the exposure surface and also cause nonuniformity of exposure as well as reduction of image definition in a semiconductor exposure device.
Therefore, in a mercury lamp filled with Ar gas or Kr gas, the present applicants have found that stable gas convection is obtained by considering the lamp shape, bringing the ratio between the length of the bulb in the axial direction and the length of the cathode which projects into the emission space within a predetermined range. Furthermore, the applicants have ascertained that with this gas convection, arc fluctuations are advantageously prevented, while at the same time, the arc fluctuations due to the temperature drop in the arc center are also advantageously prevented.
Furthermore, it has also been found that arc fluctuations can also be advantageously prevented by fixing the relationship between the length of the bulb in the axial direction, the length of the cathode which projects into the emission space, and the maximum outside diameter of the bulb in the radial direction within a predetermined range.
In the following, a bulb for a mercury lamp of the short arc type which achieves the above described stable gas convection is described.
In a bulb for a mercury lamp of the short arc type, gas convection occurs through the inner gas which receives energy from the electrodes due to the heat radiation from the electrodes and by collisions of the inner gas with the electrodes, thus resulting in the formation of a rising air flow. This rising air flow ascends upwardly along the electrodes. Since the temperature of the inner surface of the bulb is relatively lower compared to the temperature in the arc center, the risen air flow drops toward the bottom along the inner surface of the bulb.
It was found that stable operation where arc fluctuations are prevented is attained when the condition 0.3xe2x89xa6X/Lxe2x89xa60.6 is satisfied, where L is the length of the bulb in the axial direction in millimeters, and where X is the length of the cathode which projects in the axial direction into the emission space in millimeters, the direction between the anode and the cathode being defined as the axial direction.
When X/L is too large, the relative length of the anode becomes shorter. This reduces the heat radiation from the anode and therefore, the anode temperature rises. As a result, vaporization of the anode material is accelerated and blackening of the bulb occurs.
When X/L is too small, the length of the cathode which projects in the axial direction into the emission space is too low. In such a case, the air flow rising along the cathode is not adequately obtained. As a result, the air flow which descends toward the bottom along the inside of the bulb becomes stronger and predominates. Consequently the arc becomes unstable due to an unstable air flow which rises upward.
In a mercury lamp of the short arc type in accordance with the present invention, it has been found that the arc can be stabilized by fixing the relationship between the length L (mm) of the bulb in the axial direction, the length X (mm) of the cathode which projects in the axial direction into the emission space, and the maximum outside diameter D (mm) of the bulb in the radial direction in predetermined range.
The reason for this is as follows:
By fixing the relationship between D, L and X in a predetermined range, the cathode tip is located in a suitable position with respect to the diameter of the bulb, and thus, ensures that the air flow which descends along the inner surface of the bulb and the air flow which rises along the two electrodes is stable. Therefore, it can be seen how the arc is stabilized and fluctuations are reduced by the stabilized rising air flow.
When the diameter of the bulb is below a stipulated range, the electrodes of the inner surface of the bulb approach one another in relative terms. The air flow which rises along the two electrodes and the air flow which descends along the inner surface of the bulb partially collide; this causes an eddy flow and results in arc fluctuations.
As will be discussed, FIG. 7 shows specifically, the parameters for the present invention. The length of the bulb in the axial direction is labeled L, the length of the cathode which projects in the axial direction into the emission space is labeled X and the maximum outside diameter of the bulb in the radial direction is labeled D, the radial direction being defined as the direction of the cross section which is perpendicular to the axial direction. These reference labels each designate the parts shown in the drawing. Here, the length L of the bulb in the axial direction corresponds to the emission space from which the hermetically sealed portions are excluded. If the face sides of the emission space pass seamlessly into one another and can not be easily distinguished from the ends of the hermetically sealed portions, the base points of the electrodes are defined as the ends.
In the following section, the present invention is explained in detail using several different embodiments as shown in the drawings.